Hydrogen recovery by adsorbent membranes

ABSTRACT

A composite semipermeable membrane comprising microporous adsorbent material supported by a porous substrate is utilized to separate hydrogen-hydrocarbon mixtures and a sweep gas comprising some of the same hydrocarbons is passed across the low pressure side of the membrane to enhance hydrocarbon permeability. Methane is an effective sweep gas which promotes the permeation of heavier hydrocarbons even when methane is present in the membrane feed.

This application is a continuation-in-part of Ser. No. 07/923,768 filedAug. 3, 1992, now U.S. Pat. No. 5,354,547 which is acontinuation-in-part of U.S. Ser. No. 07/436,566 filed Nov. 14, 1989 andnow abandoned.

FIELD OF THE INVENTION

This invention pertains to the recovery of hydrogen fromhydrogen-hydrocarbon mixtures, and in particular the recovery ofhydrogen from such mixtures utilizing an adsorbent membrane.

BACKGROUND OF THE INVENTION

The recovery of hydrogen from hydrogen-hydrocarbon mixtures is animportant gas separation in the petroleum refining and relatedindustries. High purity hydrogen is recovered from refinery waste streamcontaining hydrogen and hydrocarbons with up to four or five carbonatom, or alternately from synthesis gas generated from natural gas bysteam-methane reforming or by partial oxidation of heavier hydrocarbons.Economical recovery of hydrogen from such streams at high purity andrecovery often requires a combination of cryogenic distillation orabsorption and pressure swing adsorption (PSA), or a combination ofdiffusion through a polymeric membrane and pressure swing adsorption(PSA). In the latter process combination, a hydrogen-rich permeate iswithdrawn from the membrane unit at a lower pressure, is compressed to ahigher pressure, and is purified by the PSA system to yield a hydrogenproduct up to 99.999 vol % purity at a pressure slightly below thehigher pressure. Hydrocarbon-rich waste streams from the membrane andPSA units often are used as fuel. U.S. Pat. Nos. 4,398,926, 4,690,695,and 4,701,187 describe various integrations of polymeric membranes andPSA systems for the recovery of hydrogen from various gas mixtures.

Prior art membrane-PSA systems for hydrogen recovery are characterizedby a large pressure differential across the membrane as hydrogenselectively diffuses, which requires initial compression to provide ahigh pressure polymeric membrane feed (typically greater than 200 psig)and recompression of the hydrogen-rich permeate as feed to the PSAsystem for final purification. These compression steps comprise asignificant portion of the capital and operating cost of a polymericmembrane-PSA system for hydrogen recovery.

U.S. Pat. No. 5,104,425 discloses a composite semipermeable membranecomprising microporous adsorptive material supported by a poroussubstrate, and teaches the use of this membrane for separation of gasmixtures including hydrogen-hydrocarbon mixtures. This membrane differsfrom conventional polymeric membranes in that the hydrocarbon impuritiespreferentially diffuse through the membrane and the hydrogen-richproduct is withdrawn as a nonpermeate stream at a pressure slightlybelow the feed pressure.

Improved methods for hydrogen recovery will be needed as the expecteddemand for hydrogen increases in the petroleum refining, transportation,and related industries. In particular, it is desirable to reducecompression cost and membrane module size when using integratedmembrane-PSA systems for hydrogen recovery. The present invention, whichutilizes an adsorbent membrane separator integrated with additional gasgeneration and separation steps as disclosed and defined in thefollowing specification and claims, addresses this need for moreefficient methods for the recovery and purification of hydrogen.

SUMMARY OF THE INVENTION

The invention is method for the recovery of hydrogen from a gaseous feedmixture comprising hydrogen and one or more hydrocarbons, in which thehydrocarbons are more strongly adsorbed on an adsorbent material thanhydrogen. The method comprises passing the feed mixture into a separatorcontaining a composite semipermeable membrane consisting essentially ofmicroporous adsorbent material supported by a porous substrate, in whichthe membrane has a feed side and a permeate side, and withdrawing fromthe separator a nonpermeate product stream enriched in hydrogen.Portions of the hydrocarbons are selectively adsorbed by the microporousadsorbent material and diffuse from the feed side to the permeate sideof the membrane as an adsorbed fluid phase. Contacting of the gaseousfeed mixture with the membrane and permeation of components through themembrane occur in the absence of chemical reaction. A permeatecomprising these hydrocarbons is withdrawn from the permeate side of themembrane.

The key feature of the invention is passing a sweep gas across thepermeate side of the membrane and withdrawing from the separator a sweepgas/permeate effluent stream, wherein the sweep gas comprises one ormore of the same hydrocarbons present in the feed mixture. The use ofthe sweep gas enhances permeation of the hydrocarbons from thehydrogen-hydrocarbon gas mixture through the adsorptive membrane.

The one or more hydrocarbons in the feed mixture include methane,ethane, ethylene, propane, propylene, butane, isobutane, butylene,isobutylene, and mixtures thereof; the sweep gas comprises one or moreof these same hydrocarbons. In one embodiment, the sweep gas is methaneor natural gas. Alternatively, the sweep gas contains hydrogen and maycontain in addition one or more of the hydrocarbons listed above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flowsheet of the present invention in which anadsorbent membrane is integrated with a pressure swing adsorptionsystem.

FIG. 2 is a schematic flowsheet of a second embodiment of the presentinvention in which an adsorbent membrane is integrated with a pressureswing adsorption system and a hydrocarbon reforming system.

FIG. 3 is a schematic flowsheet of a third embodiment of the presentinvention in which an adsorbent membrane is integrated with a pressureswing adsorption system and a hydrocarbon reforming system.

FIG. 4 is a schematic flowsheet of a fourth embodiment of the presentinvention in which an adsorbent membrane is integrated with a pressureswing adsorption system and a dephlegmator.

DETAILED DESCRIPTION OF THE INVENTION

The method of the present invention is useful for the recovery oflighter, less strongly adsorbed components in a gas mixture fromheavier, more strongly adsorbed components in the mixture. In thepreferred embodiment, the light component is hydrogen and the heaviercomponents are hydrocarbons typically including one or more of thesaturated and unsaturated C₁ -C₄ hydrocarbons, although heavierhydrocarbons, carbon oxides, and other components also may be present inthe feed mixture. High-purity hydrogen can be recovered using thismethod from gas mixtures containing from 10 to 60 vol % hydrogen.Alternatively, the method is useful for the recovery of helium frommixtures also containing C₁ -C₃ hydrocarbons and carbon oxides.

The separation method of the present invention utilizes two differentadsorption-based separation steps in series in a unique processcombination of continuous diffusion through an adsorbent membranefollowed by cyclic pressure swing adsorption (PSA). For the purposes ofthe present specification and claims, the term "primary components"refers to the component or components which permeate only slightlythrough the membrane or adsorb only slightly in the PSA adsorbers andwhich generally are recovered as the final product. The term "secondarycomponents" refers to the components which selectively andpreferentially permeate through the adsorbent membrane and areselectively and preferentially adsorbed in the PSA adsorbers. In thepreferred embodiment, a feed gas mixture containing hydrogen as theprimary component and one or more hydrocarbons as the secondarycomponents is passed over a composite semipermeable membrane whichcomprises a microporous adsorbent material supported by an essentiallyinert substrate. This membrane is generically described in the presentspecification as an adsorbent membrane, and is installed in a separatorvessel or module as described below. The hydrocarbons are selectivelyadsorbed and diffuse through the membrane in an adsorbed phase to yielda permeate enriched in these components. The less-strongly-adsorbedhydrogen is withdrawn from the membrane as a hydrogen-rich nonpermeatestream which is further purified in a pressure swing adsorption (PSA)system to yield a high-purity hydrogen product defined as at least 99.9⁺vol % hydrogen and a PSA reject stream containing essentially all of thehydrocarbons in the PSA feed along with some hydrogen. It has been foundunexpectedly in the present invention that passing themethane-containing PSA reject stream as a sweep stream across thepermeate side of the membrane increases the hydrogen recovery and thehydrocarbon permeation rates compared with the use of high purityhydrogen product from the PSA system as a sweep gas. It has been foundunexpectedly that the hydrogen flux through the membrane can be reducedby using a methane-containing sweep even though the partial pressuredriving force for hydrogen may be increased using this sweep gas. In thepresent invention, the PSA reject gas is preferred over the hydrogenproduct as the sweep gas. This results in higher hydrogen recovery and amore energy efficient separation.

Alternatively, other streams containing one or more of the samesecondary components present in the membrane feed gas may be used as asweep gas across the second membrane surface when no further separationof the membrane nonpermeate stream is desired, or when the reject streamfrom a PSA system treating membrane reject gas is used for anotherpurpose. In the separation of hydrogen-hydrocarbon mixtures, forexample, such other streams would include methane, natural gas, lighthydrocarbon mixtures optionally containing nitrogen or carbon oxides, orcertain hydrogen-hydrocarbon gas mixtures available in a petroleumrefinery. Thus an alternate embodiment of the invention comprises theseparation of a gas mixture containing primary and secondary componentsby a composite adsorptive membrane as described herein, in which themembrane is operated with a sweep gas containing one or more of the samesecondary components obtained from a source separate from the membranefeed. These secondary components can be selected from methane, ethane,ethylene, propane, propylene, butane, isobutane, butylene, isobutylene,and mixtures thereof. Typically the pressure of the membrane feed isbetween 20 and 300 psig and the pressure of the sweep gas is between 2and 50 psig.

The general criterion for selecting a sweep gas other than PSA reject isthe absence of the heavier hydrocarbons in any significant amounts. Forexample, if the membrane feed gas contains hydrogen, methane, ethane,propane, and butane, an objective of using a sweep gas could be toenhance the permeation of the heavier hydrocarbons propane and butane.If the sweep gas contains methane, ethane, propane, and butane, it ispreferable that the partial pressures of propane and butane in the sweepgas be significantly less than the partial pressures of these componentsin the membrane feed.

The enhanced permeation of hydrocarbons from a hydrogen-hydrocarbon gasmixture through an adsorptive membrane by use of a sweep gas is aphysical phenomenon determined by the interfacial properties of themembrane and the interaction of the gas mixture components with themembrane pores. The feed gas, sweep gas, and the membrane are maintainedpreferably at temperatures near or below ambient such that no chemicalreaction occurs between the gaseous components in contact with themembrane. The membrane contains no catalytic material capable ofpromoting chemical reactions in the temperature range of interest, andconsists essentially of microporous adsorbent material supported by aporous substrate.

The adsorbent membrane utilized in the present invention comprises amicroporous adsorbent material supported by an essentially inertsubstrate, wherein the substrate does not significantly affect theseparation of the gaseous components. The microporous adsorbent materialis preferably activated carbon, but others such as zeolite, activatedalumina, and silica gel may be deposited or formed on an appropriatesubstrate. An activated carbon composite membrane for use in the presentinvention and methods for making the membrane are disclosed in U.S. Pat.No. 5,104,425, the specification of which is incorporated herein byreference. One type of membrane for use in the present invention is madeby coating a porous graphite substrate with a thin film of an aqueoussuspension (latex) containing a polyvinylidine chloride-acrylateterpolymer, drying the coated substrate at 150° C. for five minutes,heating the substrate in nitrogen to 1000° C. at a rate of 1° C. perminute, holding at temperature for three hours, and cooling to ambienttemperature at 10° C. per minute. The polymer coating is carbonizedduring the heating step thereby forming an ultrathin layer ofmicroporous carbon on the substrate.

The adsorbent membrane and substrate can be fabricated in a tubularconfiguration in which the microporous adsorbent material is depositedon the inner and/or outer surface of a tubular porous substrate, and theresulting tubular adsorbent membrane elements can be assembled in ashell-and-tube configuration in an appropriate pressure vessel to form amembrane module. A plurality of membrane modules in parallel and/orseries can be utilized when gas feed rates and separation requirementsexceed the capability of a single module of practical size.Alternatively, the adsorbent membrane and support can be fabricated in aflat sheet configuration which can be assembled into a module using aplate-and-frame arrangement. Alternatively, the adsorbent membrane andsupport can be fabricated in a monolith configuration to provide a highmembrane surface area per unit volume of membrane module. The monolithcan be a porous ceramic, porous glass, porous metal, or a porous carbonmaterial. It is also possible to use a hollow fiber configuration inwhich the adsorbent membrane is supported by fine hollow fibers of thesubstrate material.

Pressure swing adsorption (PSA) systems for the separation of gasmixtures are well known in the art, and are widely used for example inthe recovery and purification of hydrogen in the petroleum refiningindustry. One representative type of PSA system is disclosed in U.S.Pat. No. 4,077,779, the specification of which is incorporated herein byreference. In a typical PSA system for the recovery of hydrogen, amixture of hydrogen and hydrocarbons is passed through one of aplurality of adsorber vessels containing one or more adsorbents such asactivated carbon or zeolite, wherein the hydrocarbons are selectivelyadsorbed by the adsorbent, and a high purity hydrogen product iswithdrawn from the adsorber. After a predetermined time period, thevessel is isolated and feed gas flows to another adsorber vessel. Theisolated vessel is depressurized to desorb adsorbed hydrocarbons, whichare withdrawn from the adsorber as a reject or waste stream. A purgestep with product hydrogen sweeps residual hydrocarbons from theadsorber for recycle to feed, or an external purge gas may be employedand the purge gas utilized externally. Various pressure equalizationsteps between adsorbers may be employed to improve product recovery andreduce power consumption in the process.

The basic embodiment of the present invention is illustrated in FIG. 1.Feed stream 1 containing one or more primary components and one or moresecondary components flows to adsorptive membrane module 101, whichcontains composite semipermeable membrane 103 which comprises anadsorbent material supported by a porous substrate as earlier described.For the purposes of this illustration, the primary component is hydrogenand the secondary components are C₁ -C₄ hydrocarbons, although other gasmixtures can be separated by the process as earlier described. These C₁-C₄ hydrocarbons preferentially adsorb and permeate through membrane 103and hydrogen-enriched nonpermeate stream, containing a residualconcentration of hydrocarbons, flows from membrane module 101. Feedstream 1 is supplied at a pressure preferably between 20 and 300 psig,and nonpermeate stream 3 is withdrawn at a slightly lower pressure dueto the pressure drop across the module of typically 2 to 5 psi. Membranenonpermeate stream 3 is compressed if necessary in compressor 105 tobetween 200 and 400 psig, and compressed stream 5 flows into PSA system107 in which essentially all of the residual hydrocarbons areselectively adsorbed. High purity hydrogen product stream 7 containingup to 99.999 vol % hydrogen is withdrawn therefrom. PSA reject stream 9is withdrawn at a pressure typically between 2 and 50 psig and passedthrough the permeate side of membrane module 101 to sweep membrane 103.Combined sweep gas/permeate effluent stream 11, containing essentiallyall hydrocarbon impurities in feed stream 1, is withdrawn from membranemodule 101 at between 1 and 45 psig. It has been unexpectedly found thatsweeping the permeate side of the membrane with the methane-containingPSA reject improves hydrogen recovery and increases the hydrocarbonpermeation rate through the membrane compared with the use of hydrogenas the sweep gas, and is thus preferred over hydrogen as a sweep gas.Membrane module 101 and PSA system 107 typically operate in anear-ambient temperature range between 60° and 100° F., although it maybe desirable to operate at the membrane module at subambienttemperature.

The present invention as illustrated by FIG. 1 is characterized by theunique feature that the hydrocarbon impurities are recovered from feedstream 1 in membrane module 101 and PSA system 107 at low pressure,while hydrogen product stream 7 and membrane nonpermeate product stream3 are recovered from the respective process steps at much higherpressures. Typically module 101 can be operated at a somewhat lowerpressure than PSA system 107 because of the high hydrocarbonpermeability of adsorbent membranes. This unique feature is in directcontrast with a typical prior art membrane-PSA combination system whichutilizes a polymeric membrane because the hydrogen selectively permeatesthrough the polymeric membrane and is withdrawn from the membrane moduleon the low pressure side. This requires significant recompression of thehydrogen-rich polymeric membrane permeate stream prior to feeding to thePSA system when compared to the present invention. In addition, feedcompression is required for polymeric membranes because of their lowabsolute permeabilities but is usually not required for adsorbentmembranes which have high permeabilities. The method of the presentinvention as illustrated in FIG. 1 therefore allows recovery of hydrogenproduct stream 7 with a much lower power consumption than acorresponding membrane-PSA combination system which uses a polymericmembrane. Module 101 and PSA system 107 can be operated at similarpressures if desired, which would eliminate the need for compressor 105.

The process configuration of FIG. 1 can be integrated with other processsteps for use in petroleum refinery applications as illustrated in thealternate embodiment of the present invention shown in FIG. 2. In thisembodiment, a steam-methane reformer for the production of hydrogen isintegrated with the embodiment of FIG. 1. A portion ofmethane-containing feed stream 12, typically natural gas or a refineryoffgas stream, is compressed if necessary to between 200 and 1000 psigby compressor 109 to yield reformer feed stream 13. A portion ofmethane-containing feedstream 12 is utilized as reformer fuel stream 15.In reformer system 111, utilizing methods well-known in the art, feedstream 13 is combined with steam (not shown) and reacted catalyticallyat an elevated temperature to form a raw synthesis gas comprisinghydrogen, carbon monoxide, carbon dioxide, water, and unreacted methane.Carbon monoxide is shifted (not shown) to form additional hydrogen andcarbon dioxide, yielding raw hydrogen product stream 19. The requiredelevated temperature is provided by combustion of reformer fuel stream15 in a reformer furnace (not shown) which yields flue gas 21. Rawhydrogen product stream 19, typically containing 15-20 vol % carbondioxide, 0-4 vol % carbon monoxide, 0-5 vol % water, and 0-4 vol %methane, is purified in PSA system 107 to yield high purity hydrogenproduct 7 for use in hydrotreating and other applications.

Feed stream 1 in this embodiment is a refinery waste stream typicallycontaining 5-60 vol % hydrogen and the remainder hydrocarbons. Suchstreams are typically used as fuel rather than for hydrogen recovery.Membrane module 101 is operated as described earlier. In the presentembodiment, compressed membrane nonpermeate product stream 5 is combinedwith raw hydrogen product stream 19, and combined stream 22 is passed toPSA system 107. In the PSA system, residual hydrocarbons, carbonmonoxide, carbon dioxide, and water are removed by selective adsorptionto yield high purity hydrogen product stream 7. Methane-containing PSAreject stream 23, which differs in composition from PSA reject stream 9of FIG. 1, is utilized as a sweep gas in membrane module 101; combinedsweep gas/permeate effluent stream 25, containing essentially allhydrocarbon impurities in feed stream 1 and impurities in raw hydrogenproduct stream 19, is withdrawn from membrane module 101 at between 2and 50 psig. The use of methane-containing sweep gas improves hydrogenrecovery and increases permeation rates through membrane 103.Optionally, a portion 27 of combined sweep gas/permeate effluent stream25 is used as fuel in reformer system 111.

This particular embodiment is useful as a retrofit to an existingrefinery reformer system 111 and PSA system 107 by installing membranemodule 101 to recover additional hydrogen from waste stream 1. Theoperating capacity of PSA system 107 would be expanded to accommodateadditional stream 5, and thus a larger volume of hydrogen product 7would be available. This retrofit would eliminate the need forinstallation of another reformer system otherwise required to increasehydrogen production capacity.

An alternate but related embodiment is illustrated in FIG. 3 in which anadsorptive membrane module, a PSA system, and a steam-methane reformerare integrated in a manner different from that of FIG. 2. In the presentembodiment of FIG. 3, methane-containing PSA reject stream 23 isutilized as fuel in reformer system 111, a portion 29 of reformer feedstream 12 is optionally reduced in pressure across valve 113, and stream31 is utilized for sweep of the permeate side of membrane 103 in module101. Combined sweep gas/permeate effluent stream 33, containingessentially all hydrocarbon impurities in feed stream 1, is withdrawnfrom membrane module 101 at between 2 and 50 psig and is compressed to200-500 psig in compressor 115. It has been unexpectedly found thatsweep with methane improves hydrogen recovery and increases thehydrocarbon permeation rate through the membrane, and is preferred overhydrogen as a sweep gas. Compressed sweep gas/permeate effluent stream37 or a portion thereof is utilized as additional feed for reformersystem 111. Alternatively, portion 39 of stream 37 can be utilized inreformer system 111 as fuel.

An alternate embodiment is illustrated in FIG. 4 in which membranemodule 101 and PSA system 107 are operated in a manner similar to theembodiment of FIG. 1. In this alternate embodiment, a portion 41 ofmethane-containing PSA reject stream 9 is utilized as a sweep gas formembrane module 101 to yield sweep gas/permeate stream 11. The remainingportion 43 of PSA reject stream 9 is removed as purge and is typicallyused as fuel. Stream 11 is compressed to 100-300 psig in compressor 117and compressed hydrocarbon-rich stream 45 is fed to dephlegmator 119,which separates stream 45 into stream 47 containing hydrocarbons lighterthan C₃, and product stream 49 containing C₃ and heavier hydrocarbons.The use of a dephlegmator for separating hydrocarbons is known in theart, and is disclosed for example in U.S. Pat. No. 4,002,042 thespecification of which is incorporated herein by reference. Adephlegmator is essentially a vertical heat exchanger having passages inwhich an upward-flowing feedstream is cooled by indirect refrigeration,thereby condensing some of the heavier mixture components which flowdownward on the walls of the passages forming a reflux liquid. Theliquid flows from the bottom of the dephlegmator as the heavier productstream; the uncondensed components are withdrawn overhead as the lightproduct.

Stream 49 is essentially liquified petroleum gas (LPG) which is a widelymarketable fuel; alternatively stream 49 can be separated further intovaluable petrochemical feedstocks. Stream 47 is combined with compressedmembrane nonpermeate gas 5 and the combined stream 51 provides feed toPSA system 107; this further processing of stream 47 maximizes therecovery of hydrogen product 7 and LPG product 49. Using the membrane torecover hydrocarbons from feed stream 1 increases the partial pressureof hydrocarbons to the dephlegmator, which allows operation of thedephlegmator at a higher temperature, which in turn requires lessrefrigeration duty. Also, use of the membrane decreases flow to thedephlegmator which reduces the size of the dephlegmator and compressor117. Thus this embodiment is a new and useful integration of anadsorptive membrane module, a PSA system, and a dephlegmator forrecovering high purity hydrogen and LPG from a refinery waste streamwhich otherwise would be used directly as a relatively low-value fuel.

EXAMPLE 1

Membranes were made by coating porous (0.7 micron pore diameter)graphite substrate discs 4.5 inches in diameter with a thin film of anaqueous suspension (latex) containing a polyvinylidine chloride-acrylateterpolymer, drying the coated discs at 150° C. for five minutes, heatingthe discs in nitrogen to 1000° C. at a rate of 1° C. per minute, holdingat temperature for three hours, and cooling to ambient temperature at10° C. per minute. The polymer coating is carbonized during the heatingstep thereby forming an ultrathin layer of microporous carbon on thesubstrate discs. A laboratory plate-and-frame membrane module wasconstructed using five of these discs having a total membrane area of0.385 sq ft. A feed gas containing 40 vol % hydrogen, 20 vol % methane,10 vol % ethane, and 10 vol % propane at 262° K. and 50 psig was passedthrough the feed side of the membrane at a flow rate of 6.74×10⁻⁵gmol/sec. Hydrogen with a purity of 99.995 vol % was passed at the sametemperature and 1 psig as a sweep gas through the permeate side of themembrane at three different flow rates corresponding to sweep:feedratios of 0.053, 0.079, and 0.155. Composition and flow rates weredetermined for the membrane nonpermeate stream and the combinedpermeate/sweep effluent stream; hydrocarbon rejection and hydrogenrecovery were calculated by material balance for each sweep:feed ratio.Hydrocarbon rejection is defined for each component as the percentage ofeach individual hydrocarbon in the membrane module feed which iswithdrawn from the membrane module in the combined permeate/sweepeffluent stream. The hydrogen recovery is defined as the differencebetween the hydrogen flow rate in the high pressure membrane product(i.e. stream 3 in FIG. 1) and the hydrogen sweep flow rate, divided bythe flow rate of hydrogen in the membrane feed (i.e. stream 1 in FIG.1). The procedures were repeated using methane (99.7 vol % purity) asthe sweep gas. The results are summarized in Table 1 and clearly showthat the use of methane as a sweep gas yields higher hydrogen recoveryand higher hydrocarbon rejection than the use of hydrogen as a sweepgas.

                                      TABLE 1                                     __________________________________________________________________________    EFFECT OF SWEEP GAS COMPOSITION ON MEMBRANE                                   PERFORMANCE                                                                             S/F = 0.053                                                                             S/F = 0.097                                                                             S/F = 0.155                                               H.sub.2                                                                            CH.sub.4                                                                           H.sub.2                                                                            CH.sub.4                                                                           H.sub.2                                                                            CH.sub.4                                             SWEEP                                                                              SWEEP                                                                              SWEEP                                                                              SWEEP                                                                              SWEEP                                                                              SWEEP                                      __________________________________________________________________________    Hydrogen Recovery                                                                       68.9 71.0 57.0 70.0 47.8 70.0                                       in Module, %                                                                  Hydrogen Product                                                                        50.0 51.5 50.9 52.4 52.3 53.2                                       Purity, Mol %                                                                 Hydrocarbon                                                                   Rejection, %                                                                  Butane    86.6 95.4 92.2 97.6 98.3 100.0                                      Propane   60.6 69.8 66.0 76.3 75.2 82.4                                       Ethane    34.5 46.8 37.6 51.9 23.0 56.4                                       Methane   20.7 29.0 8.3  29.0 23.0 27.7                                       __________________________________________________________________________

EXAMPLE 2

The data of Example 1 were used to calculate the expected performance ofthe membrane module operating in series with a pressure swing adsorption(PSA) system in which the membrane nonpermeate stream provides feed tothe PSA system as shown in FIG. 1. The performance of the PSA systemwith this feed gas was calculated using the standard design methodsdescribed by L. Lancelin et al in an article entitled "HydrogenPurification by Pressure Swing Adsorption" from the Hydrogen Symposiumof the French Association of Petrochemical Engineers, Feb. 26, 1976. Ata PSA feed concentration of about 51 vol % hydrogen (which is a typicalmembrane product purity as shown in Table 1) the PSA system operates ata hydrogen recovery of 60%. The overall hydrogen recovery from thecombined membrane-PSA system was calculated by material balance for twocases: (1) membrane sweep provided by part of the PSA hydrogen productand (2) membrane sweep provided by methane-containing PSA reject gas.The effects of sweep gas on membrane performance were based on the dataof Example 1. The results of the calculations for three differentsweep-to-feed ratios are given in Table 2 and illustrate that the use ofPSA reject as the sweep gas allows significantly higher hydrogenrecovery than the use of product hydrogen as the sweep gas.

                  TABLE 2                                                         ______________________________________                                        Overall % Hydrogen Recovery for Membrane-PSA System                                      % Hydrogen Recovery                                                Sweep/Feed Ratio                                                                           Hydrogen Sweep                                                                             PSA Reject Sweep                                    ______________________________________                                        0.053        37.3         42.6                                                0.097        23.9         42.0                                                0.155        13.8         42.0                                                ______________________________________                                    

These results in combination with Example 1 illustrate that the PSAreject stream is the preferred sweep gas because (1) themethane-containing sweep gas increases the rejection of hydrocarbons andincreases the recovery of hydrogen in the membrane module, and (2)increases the overall hydrogen recovery of the combined membrane-PSAsystem.

EXAMPLE 3

The membrane module of Example 1 was used to separate a gas stream witha composition of 20 vol % hydrogen, 20 vol % methane, 16 vol %ethane+ethylene, and 44 vol % propane+propylene, which is a typicalcomposition of a certain type of petroleum refinery waste gas stream.This gas was fed to the membrane module at 100 psig, 80° F., and a flowrate of 3.3×10⁻⁴ gmol/sec; the permeate side of the membrane was purgedwith a methane-rich sweep gas at 1 psig and 80° F. with a sweep:feedratio of 0.2. This sweep gas contained 29.6 vol % hydrogen, 10.3 vol %ethane, and 60.1 vol % methane, which is the approximate composition ofthe PSA reject stream 9 of FIG. 1. 56% of the hydrogen in the feed wasrecovered in the membrane nonpermeate at 95 psig and 80° F. at a flowrate 0.3 times the feed rate. The sweep/permeate effluent was withdrawnat 1 psig, 80° F., and a flow rate of 0.9 times that of the feed rate.Hydrocarbon recoveries in this stream were 45.4% for methane, 75.0% forethane+ethylene, and 98.0% for propane plus propylene. Thus the bulk ofthe heavier hydrocarbons permeated across the membrane while a highhydrogen recovery was achieved.

EXAMPLE 4

The experimental data of Example 3 were used to calculate the expectedperformance of the membrane module operating in series with a pressureswing adsorption (PSA) system in which the adsorbent membranenonpermeate stream provides feed to the PSA system and themethane-containing PSA reject stream provides sweep gas to the membranemodule. The feed stream of Example 3 cannot be separated using a PSAsystem alone because hydrogen recovery would be uneconomically low. Theuse of a polymeric membrane ahead of the PSA system would require a highfeed pressure because of the low hydrogen concentration in the feed gas.

The performance of the PSA system operating on the adsorbent membranenonpermeate stream was calculated using the standard design methodsdescribed by L. Lancelin et al in an article entitled "HydrogenPurification by Pressure Swing Adsorption" from the Hydrogen Symposiumof the French Association of Petrochemical Engineers, Feb. 26, 1976. Theenergy consumption for this membrane-PSA system at a hydrogen productionrate of one million standard cubic feet per day were calculated andcompared with known information for the production of hydrogen by theconventional route of steam-methane reforming (SMR) with productrecovery by PSA. The calculations were based on a hydrogen productpurity of 99.999 vol % at a pressure of 200 psig. Net energy consumptionfor operating the membrane-PSA system was calculated as the energy(fuel) content of the waste gas feed minus the energy (fuel) content ofthe combined sweep/permeate stream plus the compression energy foroperation of the membrane-PSA system. The net energy consumption for theSMR-PSA system was calculated as the energy (fuel) content of tilemethane feed plus the compression energy for operation of the SMR-PSAsystem minus the energy content of export steam from the SMR; the PSAreject stream was used as fuel in the SMR as commonly practiced. Capitalcosts were calculated for the membrane-PSA system and the SMR-PSA systemusing known commercial SMR and PSA costs and engineering estimates ofthe adsorbent membrane module cost. Table 3 summarizes the results ofthe calculations in terms of relative capital cost, hydrogen productcost, and energy consumption for the two production methods. The resultsindicate that recovery of hydrogen from waste gas containing hydrogenand hydrocarbons using the membrane-PSA system of the present inventionis less energy-intensive and more cost-effective than the production ofhydrogen from methane via steam-methane reforming when compared on thebasis of standalone membrane-PSA and SMR-PSA systems.

                  TABLE 3                                                         ______________________________________                                        Relative Costs for Standalone Membrane-PSA and SMR-PSA                        Systems                                                                              Capital Cost                                                                           Energy Cost                                                                              Net Hydrogen Cost                                  ______________________________________                                        Membrane +                                                                             0.60       0.92       0.69                                           PSA                                                                           SMR + PSA                                                                              1.00       1.00       1.00                                           ______________________________________                                    

In addition, a material balance was prepared for the key streams in themembrane-PSA system in accordance with the stream number designations ofFIG. 1, and a summary of stream flow rates, compositions, and pressuresare given in Table 4.

                  TABLE 4                                                         ______________________________________                                        STREAM FLOW RATES AND COMPOSITIONS                                            FOR EXAMPLE 4                                                                 (STREAM NUMBERS FROM FIG. 1)                                                  Stream        1      3      5    7    9    11                                 ______________________________________                                        Flowrate, MMSCFD                                                                            16.1   4.4    4.4  1.0  3.4  15.1                               Composition, vol %                                                            Hydrogen      20.1   41.1   41.1 100.0                                                                              23.9 14.8                               Methane       20.4   40.6   40.6 0.0  52.5 21.7                               Ethylene      8.4    8.1    8.1  0.0  10.5 9.0                                Ethane        7.6    6.7    6.7  0.0  8.7  8.1                                Propylene     28.5   1.7    1.7  0.0  2.1  30.4                               Propane       15.0   1.8    1.8  0.0  2.3  16.0                               Pressure, psig                                                                              100    95     205  200  5    2                                  ______________________________________                                    

Table 4 illustrates the use of PSA reject as the sweep gas in thecombined adsorbent membrane-PSA system of FIG. 1. The use of the entiremethane-rich PSA reject stream 9 as the sweep gas, rather than a portionof high purity hydrogen product stream 7, is preferred because therecovery of hydrogen is maximized. Hydrogen recovery using the PSAreject as sweep is 30.9%. A comparison of the pressures of membrane feedstream 1 and membrane nonpermeate stream 3 illustrates that significanthydrogen enrichment is achieved with minimum pressure drop, which isfundamentally different than the operation of a polymeric membrane inwhich hydrogen enrichment by permeation through the polymeric membraneis achieved with a much larger pressure drop. The high rejection ofpropane and propylene in the permeate from the adsorbent membrane isclearly illustrated by a comparison of the compositions of streams 1 and3.

The present invention thus enables the improved operation of anadsorbent membrane which comprises a microporous adsorbent materialsupported by an essentially inert substrate. By using a sweep gas otherthan high purity product gas, product recovery is increased and membraneperformance is enhanced by the increased permeation rates of thecontaminants in the membrane feed. In the recovery of hydrogen fromhydrogen-hydrocarbon mixtures, in particular when a PSA system isoperated in series with the membrane, it has been found unexpectedlythat the methane-containing reject stream from the PSA system is a moreeffective sweep gas than hydrogen product for increasing the permeationrate of the C₂ and especially C₃ hydrocarbons. This is desirable becausehydrogen recovery is significantly higher when a portion of hydrogenproduct is not used for sweep, and further because the use of thepreferred sweep gas reduces the membrane area required to process agiven gas feed rate. Methane is a particularly effective and economicalsweep gas for an adsorptive membrane system integrated with asteam-methane reformer, since a portion of the reformer methane feed canbe used for sweep with the permeate/sweep effluent gas returned to thereformer as feed or fuel.

The essential characteristics of the present invention are describedcompletely in the foregoing disclosure. One skilled in the art canunderstand the invention and make various modifications thereto withoutdeparting from the basic spirit thereof, and without departing from thescope and range of equivalents of the claims which follow.

We claim:
 1. A method for the recovery of hydrogen from a gaseous feedmixture comprising said hydrogen and one or more hydrocarbons, whereinsaid hydrocarbons are more strongly adsorbed on an adsorbent materialthan said hydrogen, said method comprising:(a) passing said feed mixtureinto a separator containing a composite semipermeable membraneconsisting essentially of microporous adsorbent material supported by aporous substrate, said membrane having a feed side and a permeate side,and withdrawing from said separator a nonpermeate product streamenriched in said hydrogen, wherein portions of said hydrocarbons areselectively adsorbed by said microporous adsorbent material and diffusefrom the feed side to the permeate side of said membrane as an adsorbedfluid phase, and further wherein contacting of said gaseous feed mixturewith said membrane and permeation of components through said membraneoccur in the absence of chemical reaction; (b) withdrawing from thepermeate side of said membrane a permeate comprising said hydrocarbons;and (c) passing a sweep gas across the permeate side of said membraneand withdrawing from said separator a sweep gas/permeate effluentstream, wherein said sweep gas comprises one or more of the samehydrocarbons present in said feed mixture.
 2. The method of claim 1wherein said feed mixture comprises one or more hydrocarbons selectedfrom the group consisting of methane, ethane, ethylene, propane,propylene, butane, isobutane, butylene, isobutylene, and mixturesthereof.
 3. The method of claim 1 wherein said sweep gas comprises oneor more hydrocarbons selected from the group consisting of methane,ethane, ethylene, propane, propylene, butane, isobutane, butylene,isobutylene, and mixtures thereof.
 4. The method of claim 3 wherein saidsweep gas comprises methane.
 5. The method of claim 1 wherein said sweepgas is natural gas.
 6. The method of claim 1 wherein said sweep gasfurther comprises hydrogen.
 7. The method of claim 1 wherein said feedmixture is supplied at a pressure between 20 and 300 psig.
 8. The methodof claim 7 wherein the pressure of said sweep gas is between 2 and 50psig.